This invention relates to a method and apparatus for fluid lancing suitable for use in removing contaminants from surfaces. The invention has particular application to removal of sludge deposits on the tube sheet of a vertical tube heat exchanger, such as in a nuclear steam generator.
A typical nuclear steam generator comprises a vertically oriented shell and a plurality of tubes disposed in the shell so as to form a tube bundle. The tubes may be of inverted U-shape or straight, depending upon the type of generator. In the former type each tube has a pair of elongated vertical portions interconnected at the upper end by a curved bight portion, so that the vertical portions of each tube straddle a center lane or passage through the tube bundle. The tubes may be dimensioned and arranged in either "square pitch" or "triangular pitch" array, so that, on each side of the center lane or passage, the vertical tube portions are disposed in a regular array of parallel rows separated by lanes and parallel columns separated by channels, with the lanes and channels intersecting each other.
A tube sheet supports the vertical portions of the tubes at their lower ends. In the case of U-shaped tubes, the vertical tube portions on one side of the center lane are connected to a primary fluid inlet plenum and those on the other side of the center lane are connected to a primary fluid outlet plenum. The primary fluid, having been heated by circulation through the reactor core, enters the steam generator through the primary fluid inlet plenum, is transmitted through the tube bundle and out the primary fluid outlet plenum. At the same time, a secondary fluid or feedwater is circulated around the tubes above the tube sheet in heat transfer relationship with the outside of the tubes, so that a portion of the feedwater is converted to steam which is then circulated through standard electrical generating equipment.
Sludge, equipment in the form of iron oxides and copper compounds along with traces of other metals, settle out of the feedwater onto the tube sheet. The sludge deposits provide a site for concentration of phosphate solution or other corrosive agents at the tube walls that can result in tube or tube sheet damage, such as pitting, corrosion, cracking, denting or thinning. Accordingly, the sludge must be periodically removed.
One known method for removal of the sludge is referred to as the sludge lance-suction method. Sludge lancing consists of using high pressure water to break up and slurry the sludge in conjunction with suction and filtration equipment that remove the water-sludge mixture for disposal or recirculation. A lance emits a high-velocity water jet or stream substantially perpendicular to the movement of the lance, i.e. parallel to the rows of tubes.
In operation, the water jet breaks up the sludge deposits and moves them toward the periphery of the tube sheet. It is desirable that the water jet have a sufficiently high velocity to dislodge the sludge deposits and move them as far as possible toward the edge of the tube sheet. However, the water velocity cannot be made too high or else it will endanger the tubes. Thus it is desirable that the water jet be effective over a maximum distance without unduly increasing the velocity of the water in the jet.